The invention resides in a method and a device for determining the moisture content and conductivity in the ground and in bulk materials by way of a capacitive sensor introduced into the ground or the bulk material which is charged by a voltage and determining the time required for the charging of the sensor.
As a reliable measure for determining the moisture content in a soil sample or a bulk material the dielectric constant εm between two electrodes is used. In comparison with many other materials water has a very high dielectric constant so that already a small moisture content of the measuring volume that is in the soil sample or in the bulk material results in a significant change of the dielectric constant εm. Typical areas of application include particularly the surveillance of the ground as well as the quality control of agricultural products (for example, flour, legumes and soya but generally also in the production and transportation of moisture-sensitive goods such as sugar.
In the determination of the moisture content of a bulk material or the ground by way of the dielectric constant εm the additional determination of the conductivity is easily possible. The electric conductivity of a material can be used for determining the presence of contaminations which noticeably influence the electric conductivity or resistance of the moist material.
Therefore the measurement of the dielectric constant εm and the conductivity of a bulk material or ground area is suitable not only for a determination of the moisture content but also for determining impurities dissolved in the water.
The following methods for the determination of the dielectric constant εm and at the second time the conductivity σm in materials are known for a characterization of soils and bulk materials:
In the so-called TDR method [1] travel differences along a wave conductor (TDR probe) are utilized which generally change with the moisture content in the material around the wave conductor. The impulse passes through one wave conductor while a second wave conductor serves as reference mass. From a measured pulse speed, the dielectric constant is calculated and subsequently the moisture content Θ of the material is calculated. An increase of the dielectric constant results in a decrease of the pulse speed. The relationship between εm and Θ must be assumed to be known. For the operation and the registration of the TDR signal a cable tester is needed which must have a resolution of several 4 GHz up to more than 100 GHz depending on the required time resolution.
[2] describes additionally the determination of the conductivity σm.
The evaluation of a TDR signal is difficult. The cable tester required and an evaluation unit with the required resolution capability for travel time measurements needed in connection therewith is very expensive.
Alternatively, in [3] a capacity measurement method is proposed which utilizes an electromagnetic oscillation circuit for determining the dielectric constant εm of the material, which is essentially less expensive than the TDR method. In this case, a sensor is provided in the form of a condenser with two condenser electrodes connected to an oscillator. An oscillator, an analog electronic circuit, generates a sinus-shaped output signal whose frequency fosz is determined by the capacity of the sensor C3, but also by other components, particularly the required inductivity L for the oscillator. The relationship is as follows:fosz=√{square root over (LC3)}=1/√{square root over (LεmCa)}  (1)
A higher dielectric constant is consequently apparent from a lower fosz:
The frequency can be determined exactly, safely and inexpensively with simple means. In this way, also accurate determinations of dielectric constants are possible by this method.
The frequency fcso generally is dependent on the conductivity of the material being examined wherein an increasing conductivity generally results in lower frequencies. A higher dielectric constant is simulated in this way. However, this influence of the conductivity becomes smaller with increasing frequency. For determining the dielectric constant therefore preferably high measuring frequencies are used. Although, the influence of the conductivity is reduced thereby, it still remains.
For a common determination of the dielectric constant εm and the conductivity σm of a material, the sensor can therefore be considered to be a complex resistor in a substitute circuit. For the determination of the complex resistance with a real and an imaginary component again a network channel analyzer is required which increases the expenses for a simultaneous determination of the moisture content and the conductivity.
It is the object of the present invention to provide a method and an apparatus for performing a method of determining the moisture content and the conductivity in soils and in bulk materials in a relatively simple and inexpensive way.